In multi-level digital data transmission systems, data in multi-level digital form is transmitted over media such as wires or fiber optic cables from a transmission line transmitter to a transmission line receiver. Multi-level signalling is widely used when high speed data needs to be transmitted through a narrow bandwidth cable, e.g., a 100 Mbit/second data transmission rate transmitted over a twisted pair cable. The multi-level data waveform is degraded with respect to its instantaneous frequency and its amplitude as it propagates along the transmission media due to electrical noise and dispersion. Electrical noise refers to the unwanted components of an electrical signal that tend to disturb accurate transmission and processing of the signal. Dispersion relates to pulse spreading of the signal and is measured in terms of time per distance traveled.
The transmission line receiver typically includes a regenerative repeater for reconstructing the transmitted multi-level data signal, regardless of electrical noise and transmission media degradations. The data signal is thereby permitted to travel through a dispersive and noisy medium, but instead of becoming more and more degraded until eventually the individual data bits are unrecognizable, the bits are repeatedly reconstructed and thus remain impervious to most of the degradation introduced by the transmission medium.
In the case of long distance, high capacity digital systems, the accuracy of the regenerative repeaters will often determine the overall effectiveness of the system. The function of the repeaters is to regenerate the transmitted data signal in its original form, ideally without error. Such reconstruction can be achieved by sampling the data signal at a regular frequency equal to the bit rate, and at each sample instant making a decision of the most probable symbol being transmitted. Typically, threshold levels are chosen to which the received signal is compared. Above each particular threshold level a binary one is registered, and below each of these thresholds a binary zero is registered. The regenerator circuit makes these zero or one decisions at times corresponding to the center of the bit intervals based on the clock information provided by a timing circuit. The center of the bit intervals generally correspond to the mid-points of the threshold level crossings of the multi-level data signal. By setting the decision times midway between the threshold level crossings, the odds of accurately reconstructing the transmitted bit are increased.
Repeaters in analog systems filter, equalize and amplify the received waveform, but are unable to reconstitute the originally transmitted waveform entirely free from degradation and noise. Signal degradation in long distance analog systems is therefore cumulative being a direct function of the number of repeater stages. In contrast, the signal degradation encountered in digital data transmission systems is purely a function of the quantization process and the system bit error rate. Hence, the possible regeneration of an exact replica of the originally transmitted waveform is a major advantage of digital transmission over corresponding analog systems.
Errors may still occur in the digital regeneration process, however, from various noises and disturbances associated with the regenerator. The noise sources can be either external of the system (for example, atmospheric noise or equipment generated noise) or internal to the system. Internal noise is present in every communications system and represents a basic limitation on the transmission and detection of signals. Hence, the amplitude of the received signal may be degraded to the point where the signal to noise ratio at the decision instant may be insufficient for an accurate decision to be made. For instance, with high noise levels, the binary zero may occur above a particular threshold and hence be registered as a binary one.
Moreover, the actual received multi-level data transmissions may be displaced in time from the true transmission. This time displacement, or intersymbol interference (ISI), of the transitions is caused by a new wave arriving at the receiver before the previous wave has reached its final value. Intersymbol interference (ISI) occurs due to pulse spreading caused by the dispersion of the transmission media. Variations in the clock rate and phase degradations (jitter) also distort the zero crossings resulting in decision time misalignment. When a pulse is transmitted in a given time slot, most of the pulse energy will arrive in the corresponding time slot at the receiver. However, because of this pulse spreading induced by the transmission medium, some of the pulse energy will progressively spread into adjacent time slots resulting in an interfering signal.
It is an object of the present invention, therefore, to provide a high-speed, low-power regeneration system for regenerating multi-level digitally transmitted data, wherein data is sampled at the transmitted data rate to compensate for both instantaneous frequency and amplitude variances.